To include XS module in dynamically linked Perl interpreters, you at least need permission to compile and install C libraries - not always a straight forward matter in corporate environments or when you are developing for a client environment rather than one you have personal control over.

The phrase "if X is installed, Y will be 20 times faster" is often a good start.

But a variable being readonly isn't a requirement for correctness. It's a development tool. So you can do something like:

BEGIN {
    if (IS_PRODUCTION) {
        eval <<'SUB';
        sub Readonly (\[$@%]@) {
            given (ref $_[0]) {
                when ('SCALAR') {${$_[0]} = $_[1]}
                when ('ARRAY') {@{$_[0]} = @_[1 .. $#_]} 
                when ('HASH') {%{$_[0]} = @_[1 .. $#_]}  
            }
        }
SUB
    }
    else {
        require Readonly;
        Readonly->import;  
    }
}
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You'll have your Readonly variables in development, and no dependency in production.